Although reference is made below essentially to mid-frequency welding transformers, the present invention is not limited thereto.
Mid-frequency transformers have been used for some time in welding methods. They are typically supplied by an inverter having a square-wave ac voltage in the range of a few kHz and a few hundred volts. The transformer outputs, to its secondary coil, a low voltage with a high current which is then rectified and used for the welding process. The high frequency with which the mid-frequency transformers are operated makes it possible to convert energy efficiently and, therefore, to use a relatively small and lightweight transformer. During operation, however, hysteresis, saturation effects, and current spikes occur, which influence the energy transfer and may have a damaging effect on components.
It is known to diminish current spikes to a partial extent by connecting diodes together in pairs. Specially selected diodes must be used for this purpose, which results in higher manufacturing and repair costs.
It is also known to diminish saturation effects via special programming of transformer controllers or a device for operating a transformer. The disadvantage of this solution is that programming of this type may only ever be used for a certain type of transformer, thereby making it necessary to employ a greater amount of programming effort when the aim is to improve various types of transformers.
The object, therefore, is to provide a method and a device for operating a transformer, which do not have the disadvantages stated above, i.e., in particular, they shorten the rise time of the input current and diminish a magnetic saturation in the transformer core and current spikes in the output current.
This object is attained via a method and a device for operating a transformer having the features of the independent claims. Advantageous embodiments are the subject matter of the subclaims and the description that follows.